I'm having similar issues. Everything is frozen when I attempt to deliver all.
I remember we used to be able to see individual message by their id, and we could click to see some header/body info, but recently, everything is invisible. CPANEL, did you do something here to hide them, why?

Any way, these kinds of things (queue building up) still happen and it's now just harder to debug. Last week it was a resolving issue, with a touch of hostname issue. Couple weeks before that, it was a blend of exceeding the box-limit size for one account.

Cpanel, I'd still like to know how I can revert to seeing specific details about the emails that are stuck in queue in "Mail Queue Manager" please advise on how I can do this and make my debugging life easier.